The present invention relates to optical devices for use in coupling light from one optical fiber to another, and to the use of such devices for optical communications.
Low insertion loss, wavelength-selective optical couplers are important components for optical fiber communications, especially for optical communications systems that use wavelength-division multiplexing (WDM). WDM systems transmit many optical channels in one fiber, with each channel being distinguished by its central wavelength. For efficient operation of a WDM communication system, the system should include the abilities to selectively add, drop and reroute channels. Ideal wavelength-selective couplers should have low insertion loss (<1 dB), high channel isolation (>30 dB), low back-reflection, and low cost. A number of wavelength-selective couplers are currently commercially available, but it is believed that none of the currently used couplers adequately meet all of the above desired characteristics.
One configuration for selecting a single channel from many channels in a fiber uses a 2×2 coupler with a fiber Bragg grating affixed to one of the coupler's outputs (e.g., Kashyap et al., Electronics Letters 26, p. 730 (1990)). Multi-wavelength input light enters one the coupler's inputs and becomes split between the two outputs. The Bragg grating at one output is chosen to reflect the desired wavelength, and light at this wavelength is emitted from the second input. This device exhibits high loss (3 dB) for all of the unselected channels and even higher loss (6 dB) for the selected channel. Such loss will typically by unacceptably high for commercial applications.
Another approach uses an optical circulator in combination with a Bragg grating at the circulator output, P. C. Becker et al., Erbium Doped Fiber Amplifiers, pp. 55-58, Academic Press (1999)). While such a configuration has a smaller insertion loss (about 2 dB), the high cost of circulators makes this device expensive.
Bilodeau et al. (Photonics Technology Letters 7, p. 388 (1995)) fabricated a fiber Mach-Zehnder interferometer that served as a wavelength-selective coupler. In order to add or drop a channel, this device relies on a precisely adjusted phase difference between two interferometer arms. This design makes the device undesirably sensitive to environmental conditions, especially temperature.
Other coupler designs use evanescent coupling of light between two fibers within a tapered region of a fused coupler. Snitzer (U.S. Pat. No. 5,457,758) used Bragg gratings to redirect the selected wavelength of light into a separate output of the coupler. Kewitsch et al. (U.S. Pat. No. 5,805,751) used a coupler with two dissimilar fibers. A Bragg grating inscribed within one of the fibers coupled light into a backward-propagating mode of the second fiber; non-resonant wavelengths were not affected and propagated with small losses through the first fiber. However, making a wavelength-selective coupler based on a fused fiber coupler requires very uniform fusion of two fibers over the length of the coupling region, making such devices difficult to manufacture.
Unlike a short-period Bragg grating, a long-period grating can couple light from a core mode into a different forward-propagating core mode (Hill et al., U.S. Pat. No. 5,216,739) or into a forward-propagating mode of the cladding (Vengsarkar, U.S. Pat. No. 5,430,817). Vengsarkar and Walker (U.S. Pat. No. 5,550,940) proposed a fused coupler-based device that uses a long-period grating. In that device, the cores of two optical fibers are spaced sufficiently far apart so that, in the absence of any gratings, there is negligible coupling of light between the two fiber cores. A single long-period grating inscribed in one fiber's core couples input light into a common cladding. A fraction of light in the cladding mode will then couple into the second fiber core provided that the interaction length is very small. This restriction limits the amount of light coupling and broadens the width of the coupling resonance.